Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
A diorite quarry we visited because of reports of lead mineralization. We didn't find any though there are lots of interesting igneous features. The Koo Koo Club was the name of a now closed nearby tavern.
|The granite here is deeply mechanically weathered, probably
due to breakdown of the amphiboles. Large boulders are relict cores of
intact granite. At left is a weathered cut with a little granite bedrock
on the quarry floor.
This granite contains occasional cavities with quartz crystals and fluorite.
Below: contemplating the rocks. There are at least five lithologies here including igneous and metamorphic rocks.
Above: There are some nice potholes here.
Left: the falls.
|Jim Falls offers a long stretch of outcrop with spectacular Archean
metamorphic rocks and equally spectacular fluvial erosion features.
Below: some outcrops are highly polished.
|Above: the rocks contain numerous potholes as well as more intricate erosional features.||Below: the Archean rocks are only moderately metamorphosed but are isoclinally folded.|
|Above: outcrop extends several kilometers upriver to the dam.
Left: old truss bridges like this, once commonplace, are now preserved as historic structures.
|Setting up camp in Hoffman Park. We were assured - cross my heart
and swear on a Bible - that the rain was over.
On this trip we linked up with the Tri-State Geological Field Conference for Saturday and Sunday.
|It rained while we were on campus registering, then at 5:30 AM it
came back for an encore, complete with thunder, lightning, high winds
and torrential rain.
Below: gathering on campus to load up.
|Above: the first stop is a nearby roadcut with Quaternary colluvium atop St. Peter Sandstone.|
|Left and below: at one end of the cut is a mass of slumped Platteville Formation intermingled with early Pleistocene glacial material. A student who did a project on this site concluded it was a channel fill but I don't see any reason it couldn't be ordinary slope failure.|
|Hard hats are a fashion must.|
|Four different members of the Platteville Formation are exposed in the quarry face.|
|Red St. Croix till overlies the older gray River Falls till.|
|Brachiopods on a bedding surface.|
|A large erratic of Keweenawan rhyolite.|
|Nice example of weathering rind in dolostone.|
|The Hastings Fault is a reactivation of the Keweenawan boundary fault and is topographically expressed by the scarp in the distance. The scarp is the west side of a linear ridge which may be a small horst.|
|West end of the Hanley Road cut.|
|The Hastings Fault. Prairie du Chien on the east (left) is juxtaposed with Jordan Sandstone on the west (right). Although it's topographically higher, the Prairie du Chien is downdropped.|
|At the entrance to the cut.|
|You never know. Maybe they're just playing possum. Or hiding behind the sofa.|
|Above: numerous paleo-karst features in the dolostone have been
filled with sediment.
Left: working our way up the cut.
|In places the dolostone is very vuggy...|
|...Or was that "buggy?"|
|View of the cuts from above.|
|Lunch was at the YMCA's Camp St. Croix.
All together now: "♫ Y - M - C - A .....♫"
|A trail leads down to Lake St. Croix, a wide stretch of the St. Croix River impounded by the Mississippi delta at Prescott.|
|Nice example of oblique waves resulting in beach drift.|
|The patches of gravel were probably formed by rips during rough water. The more vigorous return flow in the rips scoured away the finer sand.|
|A large number of springs emerge at the base of the bluffs.|
|Above: the trail is short but steep.||Below: recovering from the hike.|
|Stepping over the electrified fence.|
|Attempts to date these deposits have come up with "very old," meaning very early Pleistocene.|
|Natural or artificial? It looks exactly like concrete and the large flat surface looks artificial, but it's natural. A large sandstone slab embedded in the rock broke away to reveal the flat surface.|
|Cemented pre-Illinoisan bouldery outwash.|
|A large wedge of water-deposited sediment in the outwash.|
|Ian Williams gives his interpretation of this structure, which is probably due to collapse of an ice wedge.|
|A number of small faults offset the contact, a case of block faulting in miniature.|
|In places there are hollow iron oxide concretions, probably derived from Cretaceous rocks in northwestern Minnesota or Manitoba.|
|The stream flowing across campus is used as a natural laboratory for hydrographic studies.|
|Writing up field notes.|
|After the tempests on Friday night, Saturday night was dry and pleasant. It was still breezy, ideal for drying tents.|
|Left: foyer of the University Union.
Below: the banquet.
|Above: one of the longer field trip caravans I've been in, but not the record.||Below: here, just outside the Rock Elm structure, the rocks appear completely undisturbed.|
|One spot on the south rim of the structure offers a panoramic view. We were unlucky enough to hit rush hour. Three cars came by.|
|The Rock Elm Structure is a probable meteor impact structure about 430 million years old. The valley in the foreground more or less marks the boundary fault.|
|The highest hill is the central uplift.|
|Above: we pretty much took over anyplace we visited.
Left: Jordan Sandstone exposed in a stream bank just outside the boundary fault.
|Left: a briefing on the next stop.
Below: if you climb to the top of a rock for an overlook, you go to the bottom for an underlook, right?
|Hiking to the underlooks.|
|Left and below: syncline in the Prairie du Chien dolostone.|
|Left this anticline is obvious at stream level but not in the rocks above, possibly because of faulting.|
|As they say on the lawyer shows, we're done here.|
|There are two unique rock units within the structure that are not found outside. The Rock Elm Shale is the lower unit, one of the few true fissile shales in Wisconsin.|
|Looking west at the central uplift. Only one road even gets close to the uplift and it has no outcrop. All the outcrop is on private land.|
|This quarry was excavated for road fill and is now flooded. The shale is impervious, so the quarry is a nice illustration of a perched water table.|
|This unit, which caps the hilltops within the Rock Elm Structure, is one of my favorites because of its lovely buff color.|
|Liesgang banding in the sandstone.|
|Above: wildlife viewing.
Left: after a detour in an unsuccessful attempt to find some central uplift outcrop, we stopped at an old cemetery, where we were soon joined by a very strange local resident wondering what we were up to.
|We made one final detour to Brokaw, north of Wausau, to collect some specimens of the metasediments. The road cuts have spectacular crossbeds.|
|Left and below: some of the metaconglomerates.|
|Above: all my previous visits had been in the spring and I wasn't prepared for the jungle adjacent to the road cuts.|
Created 08 September 2006, Last Update 14 January 2020